Packed frozen sushi product and methods for preparing and for cooking the same

ABSTRACT

A packed frozen sushi product comprising a vacuumed, frozen and hermetically sealed flexible plastic bag; an open-topped plastic box placed in the bag; and a frozen sushi product contained in the box; in the plastic bag, the box and the frozen sushi product being frozen together in a unified form, the packed frozen sushi product thereby being adapted for microwave cooking. Water vapor emanating from the sushi product in thawing is allowed to flow through a space formed around the sushi material to heat a shaped rice section and a material or materials, thereby diminishing unevenness in temperature distribution after thawing. By virtue thereof, a packed frozen sushi product suitable for mass production is provided.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a 35 U.S.C. §371 national phase conversion ofInternational Application No. PCT/JP2004/012758 filed Aug. 27, 2004,which claims priority from Japanese patent application No. 2003-308141filed Aug. 29, 2003, incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a packed frozen sushi product suitablefor thawing, and a method for preparing the same, and a method forcooking the same. More particularly, the present invention relates to amicrowavable packed frozen sushi product suitable for thawing by meansof a microwave oven, and methods for preparing and for cooking the same.Further, the present invention relates to a packed frozen sushi productsuitable for thawing by means of a microwave oven, which comprises asushi product contained in a plastic packing container and vacuum-packedand hermetically sealed in the container made of a plastic film with thesushi product and the container in tight contact with each other andfrozen in a unified form, and methods for preparing and for cooking thesame.

BACKGROUND ART

For example, in the case of a rod-shaped sushi (bo-zushi), vinegaredrice is shaped, and the shaped rice section is topped with a sushimaterial or materials such as boned and prepared red sea bream, sockeyesalmon or tuna together with a thin slice of kombu kelp (shiroita kombu)cooked in a broth seasoned with vinegar, sugar and salt, and resultantis shaped into various shapes such as the rod-shape, and the shapedsushi product is wrapped with a bamboo sheath and placed in a packingbag made of a plastic film and vacuum-packed, the vacuum packed sushiproduct is frozen with a brine such as ethanol or the (vacuum-packed)shaped sushi product is frozen, and the frozen shaped sushi product isplaced in a packing bag made of a plastic film and vacuum-packed tothereby prepare a packed frozen sushi product.

Such a packed frozen sushi product by means of brine freezing isprepared by placing a sushi product in a plastic packing bag,vacuum-packing the sushi product, and brine freezing the vacuum-packedsushi product. Accordingly, no attachment of bacteria to the sushiproduct or absorption of odor from the coolant gas in the sushi productin a freezing room is caused, and therefore, the packed frozen sushiproduct is hygienically superior to, for example, a packed frozen sushiproduct prepared by vacuum-packing a sushi product posterior to freezingof the sushi product by slow freezing such as air-blast freezing,nitrogen or carbon dioxide freezing. In particular, such a packed frozensushi product frozen by brine freezing can be frozen-stored stably, forexample, over a long period of 3 months or longer when stored at astorage temperature of −18° C., or can be stored for 6 months whenstored at a storage temperature of −50° C.

However, a packed frozen sushi product which has been frozen-stored needto be thawed before being served at a meal. Methods for thawing a packedfrozen sushi product include natural thawing, running water thawing,steam thawing, hot water thawing and microwave thawing. Of these,microwave thawing is quite suitable for thawing of a packed frozen sushiproduct or the like due to its extremely short thawing time and highthawing yield. For example, in the case of above mentioned rod-shapedsushi product, natural thawing at room temperature requires a thawingtime of five hours, whereas microwave thawing enables thawing in aperiod of 2 to 5 minutes. Accordingly, microwave thawing is capable ofinstantly responding to demands and thus convenient as compared withnatural thawing at room temperature.

However, in the case of microwave thawing, calorific values derived frommicrowave absorption are different between a sushi material or materialsand a rice section, and these are also different in quantity. Further,because a surface portion is intensively heated as compared with acenter portion, a portion of the sushi material or materials is likelyto be heated to an undesirably high temperature as compared with therice section. The sushi material or materials are thereby brought intothe so-called cooked condition, and the tastes thereof are impaired.Therefore, microwave thawing has not been practically used. In a caseof, for example, a boxed sushi product or a rod-shaped sushi product, ingeneral, a rice section is preferably lukewarm, for example, the ricesection preferably has temperatures of 15 to 25° C., more preferably 20to 25° C. although this is dependent upon the type of the sushi product,whereas a sushi material or materials, for example, conger eel of eel ispreferred to have roughly the same temperatures as the rice section, forexample, temperature of 10 to 25° C., more preferably 10 to 20° C.although this is dependent upon the type of the sushi material ormaterials. However, when a packed frozen sushi product is thawed in amicrowave oven, the material or materials are heated to relatively hightemperatures to impair the taste of the sushi product. Accordingly, itis difficult to effect desired thawing by means of a microwave oven.

Under such circumstances, with a view to solving the problem that inthawing a packed frozen sushi product by means of microwave heatingusing a microwave oven or the like, a sushi material or materials of asushi product are liable to be heated to relatively high temperatures tothereby impair taste of the sushi product, the present inventors made aproposal as follows. To wrap a sushi product topped with a sushimaterial or materials with a plastic wrapping film, place the wrappedsushi product in a plastic packing bag for frozen foods, vacuum thepacking bag to vacuum-pack the sushi product, freeze the vacuum-packedsushi product en bloc by brine-freezing in a unified form to prepare aso-called packed frozen sushi product suitable for freeze-storage, andin thawing the thus prepared packed frozen sushi product by means of amicrowave oven, terminate heating by the microwave oven in a relativelyshort time so as not to raise temperatures of the sushi product toundesirably high temperatures, and using water vapor emanating from aheated rice section and prevailing in the plastic wrapping film packageduring the microwave heating and after removal of the sushi product fromthe microwave oven, heat the sushi product by steaming with the watervapor generated in the plastic wrapping film package and the residualheat of the rice section to bring a temperature of a portion where thesushi material or materials and the rice section are in contact witheach other to 15° C. or higher (see Japanese Patent publication No.3172677, and the specification of U.S. Pat. No. 5,861,184).

For preparing such a packed frozen sushi product, however, it isnecessary to tightly wrap a sushi product with a plastic wrapping film,place the wrapped sushi product in a plastic packing bag for frozenfoods, and vacuum and hermetically seal the packing bag to vacuum-packthe sushi product. This procedure should be performed carefully, andsuch a packed frozen sushi product is difficult to prepare on a largescale. Accordingly, production of such a packed frozen sushi product tomeet customers' demands involves many manual labors and much time.

It is an object of the present invention to provide a packed sushiproduct which is capable of solving the problems inherent in such aconventional frozen sushi product wrapped with a plastic film and whichis adapted for thawing by means of a microwave oven.

DISCLOSURE OF INVENTION

The present inventors have found that by placing a sushi product such asa rod-shaped sushi product in a box, placing the sushi product in aflexible packing bag together with the box containing the same, andvacuuming the packing bag, the flexible packing bag collapses underatmospheric pressure with a space left around the sushi product, and thepacking bag is hermetically sealed and an inner surface of the packingbag is brought in tight contact with the outer surfaces of the box andthe upper surface of the sushi product, and in this condition, thepacking bag is frozen, and accordingly, when the packed frozen sushiproduct is heated in a microwave oven, water vapor permeates the insideof the packing bag to efficiently heat the sushi material or materialsby steaming, enabling temperature differences in the thawed sushiproduct to be diminished. The present inventors have also found that ina sushi product which has just thawed in a microwave oven, there aretemperature differences of 40 degrees Centigrade or more between thelongitudinal ends of the sushi product and a center portion of the sushiproduct.

The present invention has been made based on these findings. It is,therefore, an object of the present invention to provide a packed frozensushi product which can be produced on a large scale and which isadapted for microwave thawing, and a method for preparing the same, anda method cooking the same.

In other words, the present invention resides in a microwavable packedfrozen sushi product comprising a vacuumed and hermetically sealedflexible microwave-safe plastic packing bag, an open-topped plastic boxplaced in the packing bag, and a sushi product placed in the box; aspace being formed in the packing bag around the sushi product, an innersurface of the packing bag being in contact with an upper surface of thesushi product, and the packing bag, the box and the sushi product beingfrozen together in a unified form. Also, the present invention residesin a method for preparing a microwavable packed frozen sushi product,the method comprising disposing a sushi material or materials on a topof a shaped rice section or in the shaped rice section to prepare asushi product, placing the prepared sushi product in a plastic box,placing the plastic box containing the sushi product in a flexiblemicrowave-safe plastic packing bag, vacuuming and hermetically sealingthe packing bag to prepare a vacuum-packed sushi product, and coolingthe vacuum-packed sushi product with a refrigerant cooled to a(predetermined) freezing temperature to freeze the packing bag, the boxand the sushi product, which are constituents of the packed sushiproduct, together in a unified form. Further, the present inventionresides in a method for cooking a microwavable packed frozen sushiproduct, the method comprising applying heat to “the microwavable packedfrozen sushi product comprising a vacuumed and hermetically sealedflexible microwave-safe plastic packing bag, an open-topped plastic boxplaced in the packing bag, and a sushi product placed in the box; theconstituents being frozen together in a unified form” by microwaveheating to thereby bring a temperature of at least a part of an exposedlongitudinal end portion of the shaped rice section to 30° C. or higher,followed by termination of the microwave heating, and allowing theresulting packed sushi product to stand for a period of 15 minutes to 1hour without unpacking to allow water vapor emanating at least from theshaped rice section to flow in a space formed between the sushi productand the box, thereby steaming the shaped rice section and the sushimaterial or materials to bring temperatures thereof between 15° C. and30° C. Moreover, the present invention resides in a method for cooking amicrowavable packed frozen sushi product, the method comprising applyingheat to “the microwavable packed frozen sushi product comprising avacuumed and hermetically sealed flexible microwave-safe plastic packingbag, an open-topped plastic box placed in the packing bag, and a sushiproduct placed in the box; the constituents being frozen together in aunified form” by microwave heating to thereby bring a temperature of anexposed longitudinal end of the shaped rice section to 30° C. or higherand bring a temperature of an exposed longitudinal end of the sushimaterial or materials to 20° C. or higher, followed by termination ofthe microwave heating, and allowing the resulting packed sushi productto stand for a period of 15 minutes to 1 hour without unpacking to allowwater vapor emanating at least from the shaped rice section to flow in aspace formed between the sushi product and the box, thereby steaming theshaped rice section and the sushi material or materials to bringtemperatures of the shaped rice section and the sushi material ormaterials between 15° C. and 30° C. In addition, the present inventionresides in a method for cooking a microwavable packed frozen sushiproduct, the method comprising applying heat to “the microwavable packedfrozen sushi product comprising a vacuumed and hermetically sealedflexible microwave-safe plastic packing bag, an open-topped plastic boxplaced in the packing bag, and a sushi product placed in the box; theconstituents being frozen together in a unified form” by microwaveheating to thereby bring a temperature of an exposed longitudinal end ofthe shaped rice section to 30° C. or higher and bring a temperature ofan exposed longitudinal end of the sushi material or materials to 20° C.or higher, followed by termination of the microwave heating, andallowing the resulting packed sushi product to stand for a period of 15minutes to 1 hour without unpacking to allow water vapor emanating atleast from the shaped rice section to flow in a space formed between thesushi product and the box, thereby steaming the shaped rice section andthe sushi material or materials to bring temperatures of the shaped ricesection and the sushi material or materials between 15° C. and 25° C. Inthe present invention, by the vacuuming in the packing procedure, thepressure in the packing bag is brought preferably to a reduced pressureof 600 mmHg or lower, preferably 400 mmHg. Further, in the presentinvention, the box is a box with no covering and may be a box-, shallowcylinder-, tray- or dish-like container having a rectangular (includingsquare), circular or oval shape when viewed in plan.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side sectional view schematically illustrating oneEmbodiment of the present invention.

FIG. 2 is a schematic front sectional view schematically illustratingthe Embodiment shown in FIG. 1.

FIG. 3 is a schematic side sectional view schematically illustratinganother Embodiment of the present invention.

FIG. 4 is a schematic front sectional view schematically illustratingthe Embodiment shown in FIG. 3.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, a sushi product means a rod-shaped sushiproduct (bo-zushi), a pressed sushi product (oshi-zushi), or a sushiproduct pressed in a box, i.e., a boxed sushi product (hako-zushi). Inthe present invention, freezing of a sushi product is performed with thesushi product placed in a box. In this procedure, the sushi product maybe placed in the box with its sushi material or materials (gu or sushineta) up and with its shaped rice section down, or the sushi product maybe placed in the box with its shaped rice section up and with its sushimaterial or materials down. In the present invention, the sushi productmay be one having a sushi material or materials as its core such as arolled sushi product (maki-zushi) or one having a rice-sandwiched sushimaterial or materials. In the present invention, the sushi product maybe placed in an appropriate microwave-transparent rigid or non-rigidplastic box. Preferably, the box is a microwave-transparent plastic box.In the present invention, the box containing the sushi product is placedin a flexible plastic packing bag. In this connection, in order tofacilitate vacuuming and cooling of the inside of the box, the boxcontaining the sushi product is placed, with no covering thereon, in theflexible plastic packing bag. The box containing the sushi product isplaced in the flexible plastic packing bag, and the packing bag isvacuumed and hermetically sealed, and the vacuumed and sealed packingbag is frozen by brine freezing, air-blast freezing, nitrogen or carbondioxide freezing or the like. In the present invention, the packedfrozen sushi product is such that the plastic box and the sushi productin the packing bag are frozen together in a unified form. In the presentinvention, the packed frozen sushi product is preferably prepared bycooling the flexible plastic packing bag which contains the boxcontaining the sushi product and which has been vacuumed andhermetically sealed to freeze the inside of the packing bag because arefrigerant does not directly contact the sushi product and becausecooling efficiency is improved. Further, in the present invention, thesushi products processed into packed frozen sushi products include sushiproducts of shaped rice topped with a sushi material or materials, forexample, a sushi product pressed in a box, i.e., a boxed sushi product,a rod-shaped sushi product, a pressed sushi product and the like, andone sushi product may be packed and frozen or a plurality of sushiproducts may be packed and frozen together.

In the packed frozen sushi product of the present invention, the box andthe packing bag are made of a microwave-transparent material, forexample, a plastic box and a plastic packing bag or the like are used.In the present invention, the plastic box for containing the sushiproduct may have such a shape as in a generally used box, and its bottomhas such a size that the sushi product formfittingly rest thereon whenthe sushi product is contained in the box, and its plane-, side-,front-, rear- and cross-sectional-shapes are so designed as tosubstantially conform the sushi product. With a view to rendering iteasy to put in and to take out the sushi product, however, it ispreferred that the plastic box for containing the sushi product be soformed as to have a top opening having a width and a length which arelarger than those of the bottom, i.e., the top opening be so formed asto be broader than the bottom. If the top opening of the box is soformed as to be broader than the bottom as described above, it isrendered easy to put in and to take out the sushi product, and yet aspace is advantageously formed around the sushi product. In the presentinvention, in a flexible packing bag which is made of a plastic film andmicrowave-safe, i.e., a flexible microwavable packing bag made of aplastic film, a space formed around the sushi product in a vacuumedcondition advantageously provides interspaces between the sushi productand the box and between the box and the packing bag during thawingbecause volumetric change is slight due to the reduced pressure in thepacking bag. In the present invention, in the vacuumed flexiblemicrowavable packing bag made of a plastic film, the interspaces betweenthe box and the sushi product and between the box and the packing bag,i.e., the space formed around the sushi product permits water vaporemanating at least from a shaped rice section of the frozen sushiproduct by heating of the frozen sushi product in thawing to circulatethrough the space. The space has a volume which enables appropriatesteaming of the sushi product after the thawing. If the space has alarge volume, the resulting product is bulky as a whole. In addition, alarger space formed around the sushi product in the packing bagundesirably results in a smaller steaming effect. On the other hand, asmaller space formed around the sushi product in the packing bagundesirably takes a longer time to effect approximately uniform steamingover the sushi product. In the present invention, a size of the spaceformed around the sushi product is 0.1 to 1, preferably 0.2 to 0.6, whenthe volume of the sushi product is supposed to be 1. In the presentinvention, the volume of the space formed around the sushi product inthe vacuumed flexible microwavable packing bag made of a plastic film isthat derived from subtracting the volume of the box and the volume ofthe sushi product from the volume of the interior of the vacuumedflexible microwavable packing bag made of a plastic film. In the presentinvention, since the shaped rice section and the sushi material ormaterials are placed in the bag made of a plastic film and hermeticallyvacuum-packed, when the shaped rice section and the sushi material ormaterials are once packed, these are optimally protected from entry ofambient air, moisture or bacteria.

In the present invention, thawing of the packed frozen sushi product isperformed, for example, by microwave-heating by means of a microwaveoven or the like. In this case, thawing of the shaped rice section ofthe sushi product is effected with heat generated by absorption ofmicrowaves with which the shaped rice section is irradiated in themicrowave oven. The thawing of the packed frozen sushi product may beeffected with the opening of the box up or with the box upside down. Ina case where the sushi material or materials may be heated, when thethawing is performed with the sushi material or materials up, the sushimaterial or materials and the shaped rice section can be heated togetherto bring temperatures of the sushi material or materials and the shapedrice section which are thawed by the microwave oven to substantially thesame levels. In a case where the sushi material or materials are desirednot to be heated by microwaves, when the thawing is performed with thesushi material or materials located under the shaped rice section,almost all microwaves in the microwave oven are applied to the shapedrice section to enable the sushi material or materials to havetemperatures lower than those of the shaped rice section after thethawing. In the present invention, when the sushi material or materialsare covered with the shaped rice section having a thickness of 1 cm ormore, preferably 2 cm or more, temperatures of the shaped rice sectionand those of the sushi material or materials can be brought,respectively, to lukewarmths and to 10° C. to 25° C., preferably 15° C.to 20° C. According to purpose of the thawing of the packed frozen sushiproduct, the packed frozen sushi product may be thawed with the openingof the box up or with the box upside down.

In the present invention, if the sushi material or materials are desirednot to be heated by microwaves, the thawing of the sushi material ormaterials is effected only with heat and water vapor which are emanatingfrom the shaped rice section or sections of the sushi product orproducts. In this case, although the thawing of the packed frozen sushiproduct is dependent upon a weight and a storage temperature of thepacked frozen sushi product, the thawing is performed, for example, asfollows. The frozen packed sushi product is subjected tomicrowave-heating in a microwave oven for 2.5 minutes to 5 minutes toeffect thawing of the frozen shaped rice section mainly, and then,without opening the packing bag, the resultant is allowed to stand, forexample in a room, for a period of 15 minutes to 1 hour, preferably 15to 45 minutes to steam the sushi material or materials with the watervapor emanating from the shaped rice section and to apply the heat ofthe shaped rice section to the sushi material or materials, whereby thethawing of the sushi material or materials is effected. In the presentinvention, in the course of the thawing, an inner surface of an upperportion of the packing bag and the sushi material or materials or theinner surface of the upper portion of the packing bag and the ricesection which have been vacuum-packed and frozen in contact with eachother become out of contact from each other, and water vapor permeatesthe resulting interspace to heat the rice section or the sushi materialor materials of the sushi product.

In the present invention, the sushi product may be placed and thawed ina microwave oven with the sushi material or materials covered with theshaped rice section. In this case, most of microwaves with which thesushi product is irradiated are absorbed in the rice section coveringthe sushi material or materials, and on the other hand, the sushimaterial or materials covered with the rice section are shielded fromthe microwaves by the rice section and thereby heated to only a smallextent. The sushi material or materials are heated with the microwavesonly in the exposed portion thereof. In a case where a boxed sushiproduct or a rod-shaped sushi product is thawed by means of a microwaveoven, a temperature distribution in the sushi product heated in themicrowave oven is not uniform. In the shaped rice section of the sushiproduct, its exposed surfaces irradiated with microwaves have hightemperatures, and portions located nearer to corners have highertemperatures because of concentration of microwave energy. According tothe temperature distribution measured by the present inventors, it wasfound that exposed longitudinal ends of the sushi material or materialsand shaped rice section of the sushi product have markedly highertemperatures as compared with those of surfaces of center portions ofthe shaped rice section and the sushi material or materials, and aninterior portion of the sushi product, for example, a portion which islocated at the center in the longitudinal direction and at which thesushi material or materials and the shaped rice section are in contactwith each other has a considerably lower temperature as compared withthose of the surfaces of the center portions of the shaped rice sectionand the sushi material or materials. In the case of a rod-shaped sushiproduct or a boxed sushi product, the rod-shaped sushi product or theboxed sushi heated by means of a microwave oven is cut into easy-to-eatsized pieces and then served. In this connection, the shaped ricesection which has just been thawed has high temperatures, and in theshaped rice section having a temperature of, for example, 40° C. orhigher, adhesion between rice grains thereof is strong. This results indifficulty in cutting pieces therefrom, and thus the sushi product islikely to undergo deformation during cutting and lose its originalshape. It was found, however, that when the temperatures of the shapedrice section are allowed to lower to those of 15° C. to 30° C.,preferably 15° C. to 25° C., the adhesion between the rice grains of theshaped rice section is thereby moderated to enable pieces to be cut fromthe sushi product with ease.

In the present invention, when the sushi product has been thawed bymicrowave-heating, to make use of residual heat in the rice and heat ofwater vapor for heating the sushi material or materials which remaincool, the resultant is allowed to stand without unpacking to steam thethawed sushi product with the water vapor generated in the packing bag,whereby temperatures of the sushi material or materials are raised and,in parallel therewith, temperatures of the shaped rice section arelowered. In the present invention, in the packing bag, a space is formedaround the shaped rice section to permit water vapor generated in thethawing to freely flow therethrough, whereby steaming is efficientlyeffected. By virtue of this, in a relatively short period of time, thetemperatures in the entire thawed shaped rice section can roughly beapproximated to render the thawed packed sushi product easy to cut intopieces. When the packed frozen sushi product is thawed by means of amicrowave oven, the packed frozen sushi product may be heated by themicrowave oven for a relatively long period of time, for example, 2.5minutes or longer, preferably 3.5 to 4.5 minutes to bring temperaturesof the end faces of the shaped rice section to 30° C. or higher, forexample, not lower than 40° C. or not lower than 50° C. at a timeimmediately after the thawing, and with the packing bag lefthermetically sealed, allowed to stand for a predetermined period of timeto steam the thawed packed sushi product, whereby the sushi product isrendered easy-to-cut to serve cut pieces. Further, the time for steamingmay be reduced by cooling the sushi product which has been thawed by amicrowave oven with water.

In the present invention, the sushi product contained in the box isplaced in the packing bag together with the box, and the packing bagcontaining the sushi product in the box is vacuumed under reducedpressure of 600 mmHg or lower, preferably 400 mmHg or lower. When thepacking bag is vacuumed, the bag collapses under atmospheric pressure,and the inner surface of the packing bag is brought in tight contactwith the shaped rice section to enable the sushi product to be heldfixedly in the box. In this manner, the packing bag is vacuumed, and theinner surface of an upper portion of the bag is thereby brought in tightcontact with the sushi material or materials and the shaped rice sectionunder atmospheric pressure, and the sushi material or materials and theshaped rice section are frozen in such a compactly held condition.Accordingly, the sushi material or materials undergo no dislocationduring the thawing. Further, in the present invention, the space isformed around the sushi product. In the vacuuming process, however, airaround the sushi product is removed, and heat transfer in the cooling isthereby promoted.

In the present invention, the box containing the sushi product may beprepared by topping the shaped rice section with the sushi material ormaterials in a sushi mold (kata) to make the sushi product, and placingthe thus prepared sushi product in the box with the sushi material ormaterials up, or by topping the shaped rice section placed in the boxwith the sushi material or materials to make the sushi product in thebox. In the present invention, however, the sushi product may be placedwith the sushi material or materials down in the box. In the presentinvention, the box which contains the sushi product is placed in theflexible plastic packing bag, and the packing bag is vacuumed andhermetically sealed to bring the inner surface of the flexible plasticpacking bag in tight contact with the sushi material or materials,whereby the sushi product are fixedly held in the box, and thevacuum-packed sushi product is brought in contact with a refrigerantcooled to a predetermined freezing temperature to freeze the contentsincluding the sushi product in the packing bag together. Accordingly,the procedure to prepare the frozen packed sushi product may beperformed almost mechanically with no substantial manual interventionand thus easily automatized to enable hygienic mass production.Accordingly, frozen packed sushi products of high quality can regularlybe produced. Further, in the present invention, the sushi product isplaced in the box, and the box containing the sushi product is placed inthe packing bag with no covering thereon, and consequently, the narrowinterspaces are formed between the box and the sushi product and betweenthe box and the packing bag to permit water vapor emanating mainly fromthe shaped rice section as a result of microwave-heating by means of amicrowave oven to flow through the thus formed interspaces, whereby heatis applied to the sushi material or materials still frozen to effectheating and thawing.

The packed frozen sushi product of the present invention comprises avacuumed and hermetically sealed flexible microwavable plastic packingbag, an open-topped plastic box placed in the packing bag, and a sushiproduct placed in the box. In the packing bag, a space is formed aroundthe sushi product, and an inner surface of the packing bag is in contactwith an upper surface of the sushi product, and the packing bag, the boxand the sushi product are frozen together in a unified form.Accordingly, when the packed frozen sushi product is thawed using amicrowave oven without unpacking, the inside of the packing bag isfilled with water vapor generated in the thawing. Subsequent to thethawing, steaming is effected by means of the water vapor generated inthe thawing to successively heat the sushi product, thereby enabling theentire sushi product to have roughly approximated temperatures in arelatively short time. Further, in the present invention, only byvacuuming the packing bag, the sushi product can be frozen with thepacking bag in contact with the upper surface of the sushi material ormaterials or the shaped rice section. Accordingly, the sushi productcontained in the plastic box can be fixedly held in the box. Asdescribed above, in the present invention, the sushi product is frozenwith the vacuumed packing bag in contact therewith. Accordingly, afterplacement of the sushi product in the plastic box, no substantialdislocations of the shaped rice section and the sushi material ormaterials are caused. By virtue of this, operating efficiency isimproved, and increased product yield is realized. Moreover, in thepresent invention, the sushi product is placed in the plastic box, andthe box containing the sushi product is placed in the plastic backingbag, and the resultant is frozen in a unified form. Accordingly, thespace is formed around the sushi product. In particular, the interspacesare formed between the sushi product and the box and between the box andthe packing bag. By circulation of water vapor generated in the thawingthrough the space, it is facilitated that the sushi product has roughlyapproximated temperatures after the steaming.

In the present invention, the sushi product is placed in the plastic boxso as to stabilize the position thereof. In a case of a sushi productcomprising a shaped rice section topped with a sushi material ormaterials, the sushi product may be placed in the plastic box with itssushi material or materials up or with its sushi material or materialsdown. When the sushi product is placed in the plastic box with its sushimaterial or materials down, in thawing by means of a microwave oven, itsshaped rice section located over the sushi material or materials isirradiated with most of microwaves to enable temperatures of the sushimaterial or materials to be kept lower than those of the shaped ricesection. In a case of a rolled sushi product, the sushi product may beplaced in the box irrespective of location of its sushi material ormaterials. In this connection, if exposed ends of the sushi material ormaterials are covered with rice of 5 mm or more, preferably 10 mm ormore in thickness, temperatures of the sushi material or materials canbe kept lower than those of the shaped rice section. The plastic boxcontaining the sushi product as described above, is placed in theflexible packing bag made of a plastic film to isolate from thesurrounding environment. The packing bag which contains the boxcontaining the sushi product is vacuumed and has its mouth sealed. Theplastic packing bag having its mouth sealed as a whole cooled and frozenin a unified form by soaking it in a refrigerant such as a brine. Inthis manner, the packed frozen sushi product can be prepared relativelysimply. Further, the packed frozen sushi product may be preparedrelatively simply by placing a frozen sushi product which has beenpreliminarily frozen in the plastic box, placing the box containing thefrozen sushi product in the flexible plastic packing bag, vacuuming thepacking bag, and sealing a mouth of the vacuumed packing bag.

Embodiments of the present invention will be described hereinbelow withreference to the accompanying drawings. It should be noted, however,that the present invention is by no means restricted by the followingdescription or Examples.

FIG. 1 is a schematic side sectional view schematically illustrating oneEmbodiment of the present invention. FIG. 2 is a schematic frontsectional view schematically illustrating Embodiment shown in FIG. 1. InFIGS. 1 and 2, the same reference numbers are used to designatecorresponding elements.

Embodiment 1

In Embodiment shown in FIGS. 1 and 2, a packed frozen sushi product 1has a microwave-transparent plastic packing bag 2 as an outer covering,and a microwave-transparent plastic box 4 containing a sushi product 3is contained in the plastic packing bag 2, and in a condition where theplastic packing bag is vacuumed, the interior of the packed frozen sushiproduct 1 and contents including the sushi product 3 having a shapedrice section 5 and a sushi material or materials 6 are frozen togetherin a unified form. In the packed frozen sushi product 1, the plasticpacking bag 2 is vacuumed and frozen in such a vacuumed condition that acenter portion 7 of its top is depressed under atmospheric pressure, andan inner surface 9 of the center portion 7 is thereby in tight contactwith an upper surface 8 of the sushi material or materials 6 of thesushi product 3. Accordingly, also in thawing, the upper surface 8 ofthe sushi material or materials 6 of the sushi product 3 is thawed incontact with the inner surface 9 of the center portion 7 of top of theplastic packing bag, and thus the shaped rice section 5 and the sushimaterial or materials 6 of the sushi product 3 in the box 4 undergo nosubstantial dislocations relative to pre-vacuuming positions thereof. Inaddition, since the interior of the packing bag is in the vacuumedcondition, when water vapor is generated by microwave-heating for about2.5 to 6 minutes, the packing bag 2 is not broken.

This Embodiment is constructed as described above, and thus the packedfrozen sushi product 1 may be prepared, for example, as follows. Ashaped rice section 5 is topped with a sushi material or materials 6 toprepare a sushi product 3, and the prepared sushi product 3 is placed ina plastic box 4. The box 4 containing the sushi product 3 is placed in apacking bag 2, and a mouth 10 of the packing bag 2 is connected to asuction port of a vacuum pump (neither of them is shown), and thepacking bag 2 is vacuumed under a pressure of, for example, 400 mmHg. Inthe vacuuming step, a top portion 7 of the packing bag 2 is depressedunder atmospheric pressure to press an upper surface 8 of the sushimaterial or materials 6, thereby stabilizing positions of the sushiproduct and the sushi material or materials in the box 4. Aftercompletion of the vacuuming, the mouth 10 of the packing bag 2 isheat-sealed to hermetically seal the packing bag. The hermeticallysealed plastic packing bag 2 is soaked in an ethanol brine refrigerantto freeze the plastic packing bag as a whole in a unified form, wherebythe packed frozen sushi product 1 is prepared. The packed frozen sushiproducts 1 thus prepared are cold-stored in an insulated storage at alow temperature of, for example, −25° C. or lower, and appropriately,they are taken out and thawed and served as sushi.

In this Embodiment, the packed frozen sushi product 1 may be thawed in amicrowave oven with its sushi material or materials 6 up. Alternatively,the packed frozen sushi product may be thawed with the shaped ricesection 5 up and the sushi material or materials 6 down, i.e., with theplastic box 4 turned upside down. When thawing is performed in amicrowave oven with the sushi material 8 up, the sushi product is onewhose sushi material or materials 6 may be heated. Microwaves areemitted toward the sushi product 3 in the vertical direction and lateraldirections to heat the upper surface 8, the front and rear end surfaces11 and the side surfaces 12 of the sushi material or materials 6 whichare exposed surfaces of the sushi material or materials. In exposedportions, the shaped rice section 5 and the sushi material or materials6 are heated to substantially the same degree immediately after thawingof the sushi product 3. The microwaves emitted toward the sushi product3 are absorbed solely through the exposed portions of the shaped ricesection 5 and the sushi material or materials 6 to heat the shaped ricesection 5 and the sushi material or materials 6. From the shaped ricesection 5 heated in this manner, water vapor is generated as thetemperature of the shaped rice section increases. The flexible plasticpacking bag 2 is filled with the generated water vapor to heat theshaped rice section 5 and the sushi material or materials 6.Consequently, the sushi product 3 is so thawed as to have substantiallyuniform temperature throughout the sushi product. In the following,specific Examples of this Embodiment will be given.

Example 1

1400 ml of seasoned vinegar (sushi vinegar) was added in 13 kg of cookedrice, followed by mixing to uniformly vinegar and the rice. Then, therice thus seasoned with vinegar was cooled to 50° C. to preparevinegar-seasoned rice, i.e., vinegared rice. To the prepared vinegaredrice, minced shiitake mushrooms which had been reconstituted with waterand then cooked in a broth seasoned with sugar, soy sauce and sweetcooking sake' (mirin) and minced gourd shavings (kampyo) which had beenreconstituted with water and then cooked in a broth seasoned with sugar,soy sauce and sweet cooking sake' (mirin) were added, followed by mixingto prepare sushi rice. On the other hand, belly-slit conger eels (anago)were cooked in a broth seasoned with sugar, soy sauce and sweet cookingsake' and cut into strips of 5.5 cm in width and 18 cm in length to makeconger eel fillet preparations as a sushi material.

Using 240 g of the sushi rice and 50 g of a conger eel filletpreparation as a sushi material, each of conger eel-topped boxed sushiproducts of 5.5 cm in width, 18 cm in length and 3 cm in thickness (Lsize) was prepared using a box-shaped sushi mold (hako-gata). The congereel-topped boxed sushi product (L size) was removed from the sushi moldand placed in a plastic box with its conger eel fillet preparation as asushi material down. The box had inner dimensions of 6.0 cm in width and19.2 cm in length at its bottom, and 7.0 cm in width and 20.5 cm inlength at its top opening, and 3.0 cm in depth. The box which containedthe sushi product was placed in a flexible packing bag made of alaminated film composed of a polyethylene film as an inner layer and anylon film as an outer layer, and the packing bag was vacuumed by avacuum pump under a pressure of 40 cmHg and had its mouth heat-sealed.In the vacuumed packing bag, its top portion was depressed and in tightcontact with the shaped rice section.

The packing bag, in which the conger eel-topped boxed sushi product hadbeen placed together with the plastic box containing the same and whichhad been vacuumed and heat-sealed, was kept in ethyl alcohol having atemperature of −35° C. as a brine for 45 minutes and therebybrine-frozen to obtain a packed frozen conger eel-topped sushi product(L size). In the packed frozen conger eel-topped sushi product (L size),the plastic packing bag as an outer covering, the plastic box and theconger eel-topped sushi product as contents were frozen together in aunified form. The packed frozen conger eel-topped sushi products (Lsize) were stored at −23.3° C. on average.

In cooking the packed frozen conger eel-topped sushi product (L size), a500 W microwave oven was used.

The packed frozen conger eel-topped sushi product (L size) in thisExample was placed in the 500 W microwave oven and thawed by applyingheat for 4 minutes. 4 minutes, which was the same as the time of thawingby the microwave oven, after completion of the thawing, the thawedpacked sushi product was removed from the microwave oven and allowed tostand at room temperature for 45 minutes to effect steaming. In thisExample, with respect to the microwave-exposed rice section of thethawed sushi product, temperatures of “an upper corner portion of theshaped rice section where three edges meet together” (in Table 1,referred to as “corner portion of the end of the upper surface of therice section”), temperatures of “a portion of the upper surface of theshaped rice section which was located 1 cm inner from the longitudinalend and at the center in the width direction” (in Table 1, referred toas “portion of the upper surface of an end portion of the rice section”)and temperatures of “a portion of the upper surface of the shaped ricesection which is located at the center in the width direction and in thelongitudinal direction” (in Table 1, referred to as “center portion ofthe upper surface of the rice section”) were measured immediately afterthe thawing, after a lapse of 30 minutes in the steaming, and after alapse of 40 minutes for the steaming. Also, with respect to the sushimaterial of the thawed sushi product, temperatures of “a portion 1cminner from the longitudinal end of the box-side surface of the sushimaterial” (in Table 1, referred to as “end portion of the box-side ofthe sushi material”), temperatures of “a center portion of the box-sidesurface of the sushi material” (in Table 1, referred to as “centerportion of the box-side surface of the sushi material”), temperatures of“a portion 1 cm inner from the longitudinal end of the vinegaredrice-side surface of the sushi material” (in Table 1, referred to as“end portion of the rice-side surface of the sushi material”) andtemperatures of “a center portion of the vinegared rice-side of thesushi material” (in Table 1, referred to as “center portion of therice-side surface of the sushi material”) were measured immediatelyafter the thawing, after a lapse of 30 minutes in the steaming, andafter a lapse of 45 minutes for the steaming. The results are shown inTable 1.

TABLE 1 Temp. ° C. post-30 post-45 immediately minutes- minutes- afterthawing steaming steaming Corner portion of the end of the 68.4 29.629.9 upper surface of the rice section Portion of side surface of an end47.6 29.6 30.7 portion of the rice section Center portion of the uppersurface 3.6 8.1 19.0 of the rice section End portion of the box-sidesurface 47.1 29.9 31.0 of the sushi material Center portion of thebox-side 28.3 13.7 18.8 surface of the sushi material End portion of therice-side surface 52.6 25.8 30.2 of the sushi material Center portion ofthe rice-side 19.9 12.8 17.5 surface of the sushi material

In the boxed sushi product, immediately after the thawing, there weredifferences of about 44° C. to 65° C. between the center portion in thelongitudinal direction and the longitudinal end portions. When the sushiproduct was steamed for 45 minutes, however, the shaped rice section andthe sushi material had a substantially uniform temperature distributionexcept the end portions, and temperatures exceeded 15° C. throughout thesushi product, and temperature differences were within a desired rangeof 15 to 30° C., and the boxed sushi product was easily cut with akitchen knife.

Example 2

Using 240 g of sushi rice and 50 g of a conger eel fillet preparation asa sushi material, each of conger eel-topped boxed sushi products of 5.5cm in width, 18 cm in length and 3 cm in thickness (L size) was preparedusing a box-shaped sushi mold (hako-gata). The sushi rice was prepared,in the same manner as in Example 1, by mixing minced shiitake mushroomsand minced gourd shavings (kampyo) each of which had been reconstitutedwith water and then cooked in a broth seasoned with sugar, soy sauce andsweet cooking sake' (mirin) with rice vinegared and cooled to 50° C. Theconger eel fillet preparation was prepared, in the same manner as inExample 1, by cooking in a broth seasoned with sugar, soy sauce andsweet cooking sake'. The conger eel-topped boxed sushi product (L size)was removed from the sushi mold and placed in a plastic box with itsconger eel fillet preparation as a sushi material down. The box had thesame inner dimensions as in Example 1, i.e., those of 6.0 cm in widthand 19.2 cm in length at its bottom, and 7.0 cm in width and 20.5 cm inlength at its top opening, and 3.0 cm in depth. The box which containedthe sushi product was placed in a flexible packing bag made of alaminated film composed of a polyethylene film as an inner layer and anylon film as an outer layer, and the packing bag was vacuumed by avacuum pump under a pressure of 40 cmHg and had its mouth heat-sealed.In the vacuumed packing bag, its top portion was depressed and in tightcontact with the shaped rice section.

The packing bag, in which the conger eel-topped boxed sushi product hadbeen placed together with the plastic box containing the same and whichhad been vacuumed and heat-sealed, was kept in ethyl alcohol having atemperature of −35° C. as a brine for 45 minutes and therebybrine-frozen to obtain a packed frozen conger eel-topped sushi product(L size). In the packed frozen conger eel-topped sushi product (L size),the plastic packing bag as an outer covering, the plastic box and theconger eel-topped sushi product as contents were frozen together in aunified form. The packed frozen conger eel-topped sushi products (Lsize) were stored at −23.6° C. on average.

In cooking the packed frozen conger eel-topped sushi product (L size), a500 W microwave oven was used.

The packed frozen conger eel-topped sushi product (L size) in thisExample was placed in the 500 W microwave oven and thawed by applyingheat for 3 minutes and 20 seconds. 3 minutes and 20 seconds, which wasthe same as the time of thawing by the microwave oven, after completionof the thawing, the thawed packed sushi product was removed from themicrowave oven and allowed to stand at room temperature for 45 minutesto effect steaming. In this Example, with respect to themicrowave-exposed rice section of the thawed sushi product, temperaturesof “an upper corner portion of the shaped rice section where three edgesmeet together” (in Table 2, referred to as Corner portion of the end ofthe upper surface of the rice section), temperatures of “a portion ofthe side surface of the shaped rice section which was located 1 cm innerfrom the longitudinal end and at the center in the width direction” (inTable 2, referred to as Portion of side surface of an end portion of therice section) and temperatures of “a portion of the upper surface of theshaped rice section which is located at the center in the widthdirection and in the longitudinal direction” (in Table 2, referred to asCenter portion of the upper surface of the rice section) were measuredimmediately after the thawing, after a lapse of 30 minutes in thesteaming, and after a lapse of 45 minutes for the steaming. Also, withrespect to the sushi material of the thawed sushi product, temperaturesof “a portion 1 cm inner from the longitudinal end of the box-sidesurface of the sushi material” (in Table 2, referred to as End portionof the box-side surface of the sushi material), and temperatures of “aportion 1 cm inner from the longitudinal end of the vinegared rice-sidesurface of the sushi material” (in Table 2, referred to as End portionof the rice-side surface of the sushi material) were measuredimmediately after the thawing, after a lapse of 30 minutes in thesteaming, and after a lapse of 45 minutes for the steaming. The resultsare shown in Table 2.

TABLE 2 Temp. ° C. post-30 post-45 immediately minutes- minutes- afterthawing steaming steaming Corner portion of the end of the 62.9 20.926.9 upper surface of the rice section Portion of side surface of an end42.4 20.7 26.4 portion of the rice section Center portion of the uppersurface 2.2 5.8 13.1 of the rice section End portion of the box-sidesurface 42.6 25.2 26.0 of the sushi material End portion of therice-side surface 44.2 25.6 26.1 of the sushi material

In the boxed sushi product, immediately after the thawing, there weredifferences of about 40° C. to 60° C. between the center portion in thelongitudinal direction of the rice section and the longitudinal endportions of the rice section. When the sushi product was steamed for 45minutes, however, the shaped rice section and the sushi material had asubstantially uniform temperature distribution except the end portions,and temperatures exceeded 15° C. throughout the sushi product, andtemperature differences were within a range of about 15° C. or less, andthe boxed sushi product was easily cut with a kitchen knife.

Example 3

Using 180 g of sushi rice and 40 g of a conger eel fillet preparation asa sushi material, each of conger eel-topped boxed sushi products of 5.5cm in width, 14 cm in length and 3 cm in thickness (S size) was preparedusing a box-shaped sushi mold (hako-gata). The sushi rice was prepared,in the same manner as in Example 1, by mixing minced shiitake mushroomsand minced gourd shavings (kampyo) each of which had been reconstitutedwith water and then cooked in a broth seasoned with sugar, soy sauce andsweet cooking sake' (mirin) with rice vinegared and cooled to 50° C. Theconger eel fillet preparation was prepared, in the same manner as inExample 1, by cooking in a broth seasoned with sugar, soy sauce andsweet cooking sake'. The conger eel-topped boxed sushi product (L size)was removed from the sushi mold and placed in a plastic box with itsconger eel fillet preparation as a sushi material down. The box had thesame inner dimensions as in Example 1, i.e., those of 6.5 cm in widthand 14.0 cm in length at its bottom, and 7.2 cm in width and 15.7 cm inlength at its top opening, and 3.3 cm in depth. The box which containedthe sushi product was placed in a flexible packing bag made of alaminated film composed of a polyethylene film as an inner layer and anylon film as an outer layer, and the packing bag was vacuumed by avacuum pump under a pressure of 40 cmHg and had its mouth heat-sealed.In the vacuumed packing bag, its top portion was depressed and in tightcontact with the shaped rice section.

The packing bag, in which the conger eel-topped boxed sushi product hadbeen placed together with the plastic box containing the same and whichhad been vacuumed and heat-sealed, was kept in ethyl alcohol having atemperature of −35° C. as a brine for 45 minutes and therebybrine-frozen to obtain a packed frozen conger eel-topped sushi product(S size). In the packed frozen conger eel-topped sushi product (S size),the plastic packing bag as an outer covering, the plastic box and theconger eel-topped sushi product as contents were frozen together in aunified form. The packed frozen conger eel-topped sushi products (Ssize) were stored at −18.4° C. on average.

In cooking the packed frozen conger eel-topped sushi product (S size), a500 W microwave oven was used.

The packed frozen conger eel-topped sushi product (S size) in thisExample was placed in the 500 W microwave oven and thawed by applyingheat for 2.5 minutes. 2.5 minutes, which was the same as the time ofthawing by the microwave oven, after completion of the thawing, thethawed packed sushi product was removed from the microwave oven andallowed to stand at room temperature for 45 minutes to effect steaming.In this Example, with respect to the microwave-exposed rice section ofthe thawed sushi product, temperatures of “a portion of the side surfaceof the shaped rice section which was located 1 cm inner from thelongitudinal end and at the center in the width direction” (in Table 3,referred to as Portion of side surface of an end portion of the ricesection) and temperatures of “a portion of the upper surface of theshaped rice section which is located at the center in the widthdirection and in the longitudinal direction” (in Table 3, referred to asCenter portion of the upper surface of the rice section) were measuredimmediately after the thawing, after a lapse of 30 minutes in thesteaming, and after a lapse of 45 minutes for the steaming. Also, withrespect to the sushi material of the thawed sushi product, temperaturesof “a portion 1 cm inner from the longitudinal end of the box-sidesurface of the sushi material” (in Table 3, referred to as End portionof the box-side surface of the sushi material), and temperatures of “aportion 1 cm inner from the longitudinal end of the vinegared rice-sidesurface of the sushi material” (in Table 2, referred to as End portionof the rice-side surface of the sushi material) were measuredimmediately after the thawing, after a lapse of 30 minutes in thesteaming, and after a lapse of 45 minutes for the steaming. The resultsare shown in Table 3.

TABLE 3 Temp. ° C. post-30 post-45 immediately minutes- minutes- afterthawing steaming steaming Portion of side surface of an end 43.0 24.523.4 portion of the rice section Center portion of the upper surface 5.014.3 17.1 of the rice section End portion of the box-side surface 30.521.3 20.5 of the sushi material End portion of the rice-side surface26.6 22.9 21.3 of the sushi material

In the boxed sushi product, immediately after the thawing, there was adifference of about 38° C. between the center portion in thelongitudinal direction of the rice section and the longitudinal endportion of the rice section. When the sushi product was steamed for 30minutes, however, temperatures of the shaped rice section and the sushimaterial exceeded 14° C. throughout the sushi product, and temperaturedifferences were within a range of about 10° C. or less. When the sushiproduct was steamed for 45 minutes, the shaped rice section and thesushi material had a substantially uniform temperature distributionexcept the end portions, and temperatures were 15° C. or higherthroughout the sushi product, and temperature differences were within arange of about 6° C. or less. In the both post-steaming cases, the boxedsushi product was easily cut with a kitchen knife.

Example 4

Using 240 g of sushi rice and 50 g of a conger eel fillet preparation asa sushi material, each of conger eel-topped boxed sushi products of 5.5cm in width, 18 cm in length and 3 cm in thickness (L size) was preparedusing a box-shaped sushi mold (hako-gata). The sushi rice was prepared,in the same manner as in Example 1, by mixing minced shiitake mushroomsand minced gourd shavings (kampyo) each of which had been reconstitutedwith water and then cooked in a broth seasoned with sugar, soy sauce andsweet cooking sake' (mirin) with rice vinegared and cooled to 50° C. Theconger eel fillet preparation was prepared, in the same manner as inExample 1, by cooking in a broth seasoned with sugar, soy sauce andsweet cooking sake'. The conger eel-topped boxed sushi product (L size)was removed from the sushi mold and placed in a plastic box with itsconger eel fillet preparation as a sushi material up. The box had innerdimensions of 6.0 cm in width and 19.2 cm in length at its bottom, and7.0 cm in width and 20.5 cm in length at its top opening, and 3.3 cm indepth. The box which contained the sushi product was placed in aflexible packing bag made of a laminated film composed of a polyethylenefilm as an inner layer and a nylon film as an outer layer, and thepacking bag was vacuumed by a vacuum pump under a pressure of 40 cmHgand had its mouth heat-sealed. In the vacuumed packing bag, its topportion was depressed and in tight contact with the shaped rice section.

The packing bag, in which the conger eel-topped boxed sushi product hadbeen placed together with the plastic box containing the same and whichhad been vacuumed and heat-sealed, was kept in ethyl alcohol having atemperature of −35° C. as a brine for 45 minutes and therebybrine-frozen to obtain a packed frozen conger eel-topped sushi product(L size). In the packed frozen conger eel-topped sushi product (L size),the plastic packing bag as an outer covering, the plastic box and theconger eel-topped sushi product as contents were frozen together in aunified form. The packed frozen conger eel-topped sushi products (Lsize) were stored at −18.0° C. on average.

In cooking the packed frozen conger eel-topped sushi product (L size), a500 W microwave oven was used.

The packed frozen conger eel-topped sushi product (L size) in thisExample was placed in the 500 W microwave oven and thawed by applyingheat for 4 minutes and 15 seconds. 4 minutes and 15 seconds, which wasthe same as the time of thawing by the microwave oven, after completionof the thawing, the thawed packed sushi product was removed from themicrowave oven and allowed to stand at room temperature for 45 minutesto effect steaming. In this Example, with respect to themicrowave-exposed rice section of the thawed sushi product, temperaturesof “a portion of the side surface of the longitudinal end of the shapedrice section” (in Table 4, referred to as End side portion of the ricesection) and temperatures of “a portion of the lower surface of theshaped rice section which is located at the center in the widthdirection and in the longitudinal direction” (in Table 4, referred to asCenter portion of the lower surface of the rice section) were measuredimmediately after the thawing, after a lapse of 30 minutes in thesteaming, and after a lapse of 45 minutes for the steaming. Also, withrespect to the sushi material of the thawed sushi product, temperaturesof “a portion 1 cm inner from the longitudinal end of the upper side ofthe sushi material” (in Table 4, referred to as End side portion of theupper side of the sushi material), and temperatures of “a portion 1 cminner from the longitudinal end side of the rice-side, i.e., the lowerside of the sushi material” (in Table 4, referred to as End side portionof the rice-side of the sushi material) were measured immediately afterthe thawing, after a lapse of 20 minutes in the steaming, and after alapse of 30 minutes for the steaming. The results are shown in Table 4.

TABLE 4 Temp. ° C. post-20 post-30 immediately minutes- minutes- afterthawing steaming steaming End side portion of the rice section 58.2 30.629.1 Center portion of the lower surface 27.8 26.1 25.6 of the ricesection End side portion of the upper side 23.4 24.5 26.2 of the sushimaterial End side portion of the rice-side 18.6 21.7 23.3 of the sushimaterial

In the boxed sushi product, immediately after the thawing, there was adifference of about 30° C. between the center portion in thelongitudinal direction of the rice section and the longitudinal endportion of the rice section. When the sushi product was steamed for 20or 30 minutes, however, the shaped rice section and the sushi materialhad a substantially uniform temperature distribution except the endportions, and temperatures exceeded 20° C. throughout the sushi product,and temperature differences were within a range of about 10° C. or less,and the boxed sushi product was easily cut with a kitchen knife.

Example 5

1400 ml of seasoned vinegar (sushi vinegar) was added in 13 kg of cookedrice, followed by mixing to uniformly vinegar the rice. Then, the ricethus seasoned with vinegar was cooled to 50° C. to preparevinegar-seasoned rice, i.e. vinegared rice. 210 g of the preparedvinegared rice was spread out and 40 g of core was placed on top of thespread out vinegared rice to obtain each of round bar shapedthick-rolled sushi products of 5 cm in diameter and 14.5 cm in length (Ssize) using a sushi rolling machine for thick-rolled sushi or the like.In this Example, the following ingredients were used as the core; 1)finely chopped shiitake mushrooms which had been reconstituted withwater and then cooked in a broth seasoned with sugar, soy sauce andsweet cooking sake' (mirin); 2) finely chopped gourd shavings (kampyo)which had been reconstituted with water and then cooked in a brothseasoned with sugar, soy sauce and sweet cooking sake' (mirin); 3) Eggseasoned with stock and seasonings such as sugar and soy sauce andcooked into box-shaped Japanese thick omelet, which was then cut into astick-shape with 1 cm width, 1 cm height and 14.5 cm length; and 4)Boiled Japanese mitsuba herb cut to appropriate length.

The thick-rolled sushi product (S size) thus prepared was placed in aplastic box. The box had inner dimensions of 6.5 cm in width and 14.0 cmin length at its bottom, and 7.2 cm in width and 15.7 cm in length atits top opening, and 3.3 cm in depth. The box which contained the sushiproduct was placed in a flexible packing bag made of a laminated filmcomposed of a polyethylene film as an inner layer and a nylon film as anouter layer, and the packing bag was vacuumed by a vacuum pump under apressure of 40 cmHg and had its mouth heat-sealed. In the vacuumedpacking bag, its top portion was depressed and in tight contact with thetop surface of the thick-rolled sushi product.

The packing bag, in which the thick-rolled sushi product had been placedtogether with the plastic box containing the same and which had beenvacuumed and heat-sealed, was kept in ethyl alcohol having a temperatureof −35° C. as a brine for 45 minutes and thereby brine-frozen to obtaina packed frozen thick-rolled sushi product (S size). In the packedfrozen thick-rolled sushi product (S size), the plastic packing bag asan outer covering, the plastic box and the thick-rolled sushi product ascontents were frozen together in a unified form. The packed frozenthick-rolled sushi products (S size) were stored at −24.3° C. onaverage.

In cooking the packed frozen thick-rolled sushi product (S size), a 500W microwave oven was used.

The packed frozen thick-rolled sushi product (S size) in this Examplewas placed in the 500 W microwave oven and thawed by applying heat for 3minutes and 20 seconds. 3 minutes and 20 seconds, which was the same asthe time of thawing by the microwave oven, after completion of thethawing, the thawed packed sushi product was removed from the microwaveoven and allowed to stand at room temperature for 45 minutes to effectsteaming. In this Example, with respect to the microwave-exposed riceportion of the thawed thick-rolled sushi product, temperatures of “aportion located 1 cm inner from the longitudinal end” (in Table 5,referred to as Portion of side surface of an end portion of the ricesection), temperatures of “a portion of the surface at the center in thelongitudinal direction” (in Table 5, referred to as Center portion ofthe upper surface of the rice section) and temperatures of “a portionlocated 2mm in depth from the surface at the center in the longitudinaldirection” (in Table 5, referred to as Center portion inside the ricesection) were measured immediately after the thawing, after a lapse of30 minutes in the steaming, and after a lapse of 45 minutes for thesteaming. Also, with respect to the sushi material of the thawedthick-rolled sushi product, temperatures of “a portion located 25 mm indepth from the surface at the center in the longitudinal direction” (inTable 5, referred to as The sushi materials section) were measuredimmediately after the thawing, after a lapse of 30 minutes in thesteaming, and after a lapse of 45 minutes for the steaming. The resultsare shown in Table 5.

TABLE 5 Temp. ° C. post-30 post-45 immediately minutes- minutes- afterthawing steaming steaming Portion of side surface of an end 57.3 27.726.9 portion of the rice section Center portion of the upper surface11.4 20.3 18.9 of the rice section Center portion inside the rice 5.519.9 18.0 section The sushi materials section −2.5 20.0 18.0

In the thick-rolled sushi product, immediately after thawing, there weredifferences of about 45° C. between the center portion in thelongitudinal direction and the longitudinal end portions of the shapedrice portion. 30 minutes and 45 minute after leaving the sushi productto steam, the temperatures of the shaped rice section and the sushimaterial were both generally 20° C. or more, although with somedifferences, and the overall temperature difference was within about 10°C. or less. When the sushi product was steamed for 45 minutes,temperatures of the shaped rice portion and the sushi material werewithin a temperature range of about 15 to 30° C. throughout the sushiproduct, enabling the thawed thick-roll sushi product to be cut easilywith a kitchen knife.

Example 6

In the same manner as in Example 5, vinegared rice and core wereprepared. 265 g of the prepared vinegared rice was spread out and 50 gof the core was placed on top of the spread out vinegared rice toprepare each of round bar shaped thick-rolled sushi products of 5 cm indiameter and 18 cm in length (L size) using a sushi rolling machine forthick-rolled sushi or the like. The thick-rolled sushi product (L size)thus prepared was placed in a plastic box. The box had inner dimensionsof 6.0 cm in width and 19.2 cm in length at its bottom, and 7.2 cm inwidth and 20.5 cm in length at its top opening, and 3.3 cm in depth. Thebox which contained the sushi product was placed in a flexible packingbag made of a laminated film composed of a polyethylene film as an innerlayer and a nylon film as an outer layer, and the packing bag wasvacuumed by a vacuum pump under a pressure of 40 cmHg and had its mouthheat-sealed. In the vacuumed packing bag, its top portion was depressedand in tight contact with the top surface of the thick-rolled sushiproduct.

The packing bag, in which the thick-rolled sushi product had been placedtogether with the plastic box containing the same and which had beenvacuumed and heat-sealed, was kept in ethyl alcohol having a temperatureof −35° C. as a brine for 45 minutes and thereby brine-frozen to obtaina packed frozen thick-rolled sushi product (L size). In the packedfrozen thick-rolled sushi product (L size), the plastic packing bag asan outer covering, the plastic box and the thick-rolled sushi product ascontents were frozen together in a unified form. The packed frozenthick-rolled sushi products (L size) were stored at −19.5° C. onaverage.

In cooking the packed frozen thick-rolled sushi product (L size), a 500W microwave oven was used.

The packed frozen thick-rolled sushi product (L size) in this Examplewas placed in the 500 W microwave oven and thawed by applying heat for 3minutes and 20 seconds. 3 minutes and 20 seconds, which was the same asthe time of thawing by the microwave oven, after completion of thethawing, the thawed packed sushi product was removed from the microwaveoven and allowed to stand at room temperature for 45 minutes to effectsteaming. In this Example, with respect to the microwave-exposed riceportion of the thawed thick-rolled sushi product, temperatures of “aportion located 1 cm inner from the longitudinal end” (in Table 5,referred to as Portion of side surface of an end portion of the ricesection), temperatures of “a portion of the surface at the center in thelongitudinal direction” (in Table 5, referred to as Center portion ofthe upper surface of the rice section) and temperatures of “a portionlocated 2 mm in depth from the surface at the center in the longitudinaldirection” (in Table 5, referred to as Center portion inside the ricesection) were measured immediately after the thawing, after a lapse of30 minutes in the steaming, and after a lapse of 45 minutes for thesteaming. Also, with respect to the sushi material of the thawedthick-rolled sushi product, temperatures of “a portion located 25 mm indepth from the surface at the center in the longitudinal direction” (inTable 5, referred to as The sushi materials section) were measuredimmediately after the thawing, after a lapse of 30 minutes in thesteaming, and after a lapse of 45 minutes for the steaming. The resultsare shown in Table 6.

TABLE 6 Temp. ° C. post-30 post-45 immediately minutes- minutes- afterthawing steaming steaming Portion of side surface of an end 65.2 25.923.3 portion of the rice section Center portion of the upper surface12.2 13.0 15.3 of the rice section Center portion inside the rice 10.612.2 13.4 section The sushi materials section 3.0 3.7 11.1

In the thick-rolled sushi product, immediately after thawing, there weredifferences of about 53° C. between the center portion in thelongitudinal direction and the longitudinal end portions of the shapedrice portion. 45 minutes after leaving the sushi product to steam, thetemperatures of the shaped rice section and the sushi material exceeded10° C., and the overall temperature difference was within a range ofabout 10° C. or less, enabling the thawed thick-roll sushi product to becut easily with a kitchen knife.

Example 7

1400 ml of seasoned vinegar (sushi vinegar) was added in 13 kg of cookedrice, followed by mixing to uniformly vinegar the rice. Then, the ricethus seasoned with vinegar was cooled to 50° C. to preparevinegar-seasoned rice, i.e., vinegared rice. On the other hand, back orbelly-slit eels were grilled while being repeatedly coated with a thicksauce (tare) prepared from seasoning including soy sauce, sugar andsweet cooking sake' (mirin) to prepare grilled eel fillets (kabayaki),and the grilled eel fillets were cut into pieces having dimensions of 4cm in width, 6 cm in length and 3 mm in thickness and a weight ofapproximately log per piece as sushi material pieces. 180 g of vinegaredrice was placed in a box-shaped sushi mold with dimensions of 5.5 cm inwidth, 14 cm in length and 3 cm in depth for pressed sushi making, and 4pieces (40 g) of the grilled eel as a sushi material were placed on topof the vinegared rice to produce each of eel-topped boxed sushi products(S size). The eel-topped boxed sushi product of 5.5 cm in width, 14 cmin length and 3.3 cm in thickness (S size) was removed from thebox-shaped sushi mold and placed in a plastic box with its pieces of eelas a sushi material down. The box had inner dimensions of 6.5 cm inwidth and 14.0 cm in length at its bottom, and 7.2 cm in width and 15.7cm in length at its top opening, and 3.3 cm in depth. The box whichcontained the sushi product was placed in a flexible packing bag made ofa laminated film composed of a polyethylene film as an inner layer and anylon film as an outer layer, and the packing bag was vacuumed by avacuum pump under a pressure of 40 cmHg and had its mouth heat-sealed.In the vacuumed packing bag, its top portion was depressed and in tightcontact with the shaped rice section.

The packing bag, in which the eel-topped boxed sushi product had beenplaced together with the plastic box containing the same and which hadbeen vacuumed and heat sealed, was kept in ethyl alcohol having atemperature of −35° C. as a brine for 45 minutes and therebybrine-frozen to obtain a packed frozen eel-topped sushi product (Ssize). In the packed frozen eel-topped sushi product (S size), theplastic packing bag as an outer covering, the plastic box and theeel-topped sushi product as contents were frozen together in a unifiedform. The packed frozen eel-topped sushi products (S size) were storedat −21.6° C. on average.

In cooking the packed frozen eel-topped sushi product (S size), a 500 Wmicrowave oven was used.

The packed frozen eel-topped sushi product (S size) in this Example wasplaced in the 500 W microwave oven and thawed by applying heat for 2.5minutes. 2.5 minutes, which was the same as the time of thawing by themicrowave oven, after completion of the thawing, the thawed packed sushiproduct was removed from the microwave oven and allowed to stand at roomtemperature for 45 minutes to effect steaming. In this Example, withrespect to the microwave-exposed rice portion of the thawed sushiproduct, temperatures of “a portion of the side surface of the shapedrice section which was located on the longitudinal end” (in Table 7,referred to as Portion of side surface of an end portion of the ricesection) and the temperatures of “a portion of the upper surface of theshaped rice section (in the box) which is located at the center in thewidth direction and in the longitudinal direction” (in Table 7, referredto Center portion of the upper surface of the rice section) weremeasured immediately after the thawing, after a lapse of 30 minutes inthe steaming, and after a lapse of 45 minutes for the steaming. Also,with respect to the sushi material of the thawed sushi product,temperatures of “a portion 1 cm inner from the longitudinal end of thelower surface (on the bottom of the box), i.e., box-side surface of thesushi material” (in Table 7, referred to as Surface of the lower sideend portion of the sushi material) and temperatures of “a portion 1 cminner from the longitudinal end of the vinegared rice-side surface ofthe sushi material” (in Table 7, referred to as End portion of thevinegared rice-side surface of the sushi material) were measuredimmediately after the thawing, after a lapse of 20 minutes in thesteaming, and after a lapse od 30 minutes for the steaming. The resultsare shown in Table 7.

TABLE 7 Temp. ° C. post-30 post-45 immediately minutes- minutes- afterthawing steaming steaming Portion of side surface of an end 57.0 25.723.3 portion of the rice section Center portion of the upper surface 8.219.4 20.2 of the rice section Surface of the lower side end 16.7 20.320.6 portion of the sushi material End portion of the vinegared 15.820.1 20.3 rice-side surface of the sushi material

In the boxed sushi product, immediately after the thawing, there was adifference of about 49° C. between the upper center portion in thelongitudinal direction of the rice section and the portion of sidesurface of an end portion of the rice section. When the sushi productwas steamed for 30 and 45 minutes, however, temperatures of the shapedrice section and the sushi material exceeded 15° C. in both thepost-steaming cases, and the shaped rice section and the sushi materialhad a substantially uniform temperature distribution, and temperaturedifferences were within a range of about 10° C. or less throughout thesushi product, and the boxed sushi product was easily cut with a kitchenknife.

In FIGS. 3 and 4, Embodiment is shown which is different from theEmbodiment shown in FIGS. 1 and 2. In the Embodiment shown in FIGS. 3and 4, a box 4 is made of a polypropylene. As shown in FIG. 4, the box 4has a reversed semicylindrical shape, i.e., reversed kamaboko-likeshape. This shape is so designed as to substantially conform with theshape of the sushi product placed on the bottom of the box 4. In otherwords, the front end of the box 4 has a reversed semicircular shape,i.e., cross-sectional reversed kamaboko shape. This shape is so designedas to substantially conform with the cross-sectional shape of the sushiproduct placed on the bottom of the box 4. In this Embodiment, theplastic box 4 for containing a packed frozen sushi product 1 is formedwith 7 ribs 13 along each of inner walls which inward extend from theinner walls with a view to reinforcing the box 4 and maintaining theshape of the box 4. The sushi product may be contained in the box 4 insuch a manner that it is placed on the ribs 13 and through spaces 14between the ribs 13 with its sushi material or materials 6 down andshaped rice section 5 thereon. In this connection, the sushi product maybe provided with cut lines so as to be easily separable into piecesafterward. Alternatively, the sushi product may be cut into pieces andeach of the pieces may be placed in the space 14 between the ribs 13with each rib 13 interposed between pieces. In this Embodiment, thesushi product 8 may be a rod-shaped sushi product or boxed sushiproduct, and the sushi product 8 is contained in the box 4 in such amanner that it is preliminarily provided with cut lines 15 so as to beeasily separable into 8 pieces afterward and placed on the ribs 13. FIG.3 shows, using cutting lines 16, the ribs 13 and the spaces 14 betweenthe ribs with no sushi product contained in the box. Of course, in thisEmbodiment, the box may be used as a container for a rod-shaped sushiproduct or boxed sushi product which is provided with no cut lines. Inthis Embodiment, the ribs are provided on the inside of the box.However, the ribs may be provided on the outside of the box.

Example 8

Finely chopped shiitake mushrooms which had been reconstituted withwater and then cooked in a broth seasoned with sugar, soy sauce andsweet cooking sake' (mirin) and finely chopped gourd shavings (kampyo)which had been reconstituted with water and then cooked in a brothseasoned with sugar, soy sauce and sweet cooking sake' (mirin) wereadded to vinegared rice prepared as in Example 1 to prepare sushi rice.On the other hand, belly-slit conger eels (anago) were cooked in a brothseasoned with sugar, soy sauce and sweet cooking sake' and cut intostrips of 5.5 cm in width and 18 cm in length to make conger eel filletpreparations as a sushi material.

Using 220 g of the sushi rice and 40 g of the conger eel filletpreparation as a sushi material, each of conger eel-topped boxed sushiproducts of 5.5 cm in width, 18 cm in length and 2.7 cm in thickness (Lsize) was prepared using a box-shaped sushi mold (hako-gata). The congereel-topped boxed sushi product (L size) was removed from the sushi mold,and was provided with cut lines so as to be easily separable into 8pieces afterward, and was placed in a plastic box having a reversedsemi-cylindrical shape with its conger eel fillet preparation as a sushimaterial down. The box had inner dimensions of 6.2 cm in width and 18.2cm in length in its bottom, and 6.2 cm in width and 19.2 cm in length inits top opening, and 3.0 cm in depth. The box which contained the sushiproduct was placed in a flexible packing bag made of a laminated filmcomposed of a polyethylene film as an inner layer and a nylon film as anouter layer, and the packing bag was vacuumed by a vacuum pump under apressure of 40 cmHg and had its mouth heat-sealed. In the vacuumedpacking bag, its top portion was depressed and in tight contact with theshaped rice section. Thus vacuumed and heat-sealed packing bagcontaining the conger eel-topped boxed sushi product (L size) was keptin ethyl alcohol having a temperature of −35° C. as a brine for 45minutes and thereby brine-frozen to obtain a packed frozen congereel-topped sushi product (L size).

In the packed frozen conger eel-topped sushi product (L size), theplastic packing bag as an outer covering, the plastic box and the congereel-topped sushi product as contents were frozen together in a unifiedform. The packed frozen conger eel-topped sushi products (L size) werestored at −23.4° C. on average.

In cooking the packed frozen conger eel-topped sushi product (L size), a500 W microwave oven was used. The packed frozen conger eel-topped sushiproduct (L size) in this Example was placed in the 500 W microwave ovenand thawed by applying heat for 3 minutes and 10 seconds. 3 minutes and10 seconds, which was the same as the time of thawing by the microwaveoven, after completion of the thawing, the thawed packed sushi productwas removed from the microwave oven and allowed to stand at roomtemperature of 25° C. for 30 minutes to effect steaming. In this processof steaming, temperatures of given portions were measured immediatelyafter the thawing, after a lapse of 15 minutes in the steaming (in theTable, referred to simply as “15 minutes later”), and after a lapse of30 minutes for the steaming (in the Table, referred to simply as “30minutes later”). The results are shown in Table 8.

In Table 8, “Portion of side surface of an end portion of the ricesection” represents the temperatures of a corner portion in the shapedrice section, where three edges of the sides not in contact with thesushi material meet together. Also, “Center portion of the upper surfaceof the rice section” represents the temperatures of a portion at thecenter in the longitudinal direction and in the width direction of theupper surface of the sides not in contact with the sushi material. Also,“End portion of the box-side surface of the sushi material” representsthe temperatures of a portion located 1 cm inner from the end of thesushi material surface in contact with the bottom of the plasticcontainer. Further, “Center portion of the upper surface of the sushimaterial” represents temperatures of a portion at the center in thelongitudinal direction and in the width direction of the sushi materialsurface in contact with the bottom of the plastic container.Furthermore, “End portion of the rice-side surface of the sushimaterial” represents temperatures of a portion located 1 cm inner fromthe end of the sushi material surface in contact with the rice section.

TABLE 8 Temp. ° C. post-15 post-30 immediately minutes- minutes- afterthawing steaming steaming Portion of side surface of an end 62.9 36.123.3 portion of the rice section Center portion of the upper surface38.0 10.7 16.5 of the rice section End portion of the box-side surface43.9 21.7 22.5 of the sushi material Center portion of the upper surface43.9 7.2 13.4 of the sushi material End portion of the rice-side surface15.8 21.8 22.0 of the sushi material

In the packed sushi product, immediately after thawing, there was adifference of about 25° C. between the temperatures of the centerportion of the upper surface of the rice section and the portion of sidesurface of an end portion of the rice section. 30 minutes after leavingthe sushi product to steam, however, the temperatures of the shaped ricesection and the sushi material were above 13° C. or higher, andtemperature distribution in the sushi product became roughly uniform,and the overall temperature differences were within a range of about 10°C. or less, enabling the thawed boxed-sushi product to be cut easilywith a kitchen knife.

Also, in this Example, the sushi product was sufficiently thawed after30 minutes of steaming time, however, when the sushi product was closelywrapped with a wrapping film so that the volume ratio of the space tothe sushi product was less than 0.1, 45 minutes of steaming time orlonger was required, indicating that there was a reduction of 15 minutesor more in the steaming time required.

Example 9

210 g of vinegared rice prepared as in Example 5 was spread on a sheetof nori seaweed and 40 g of core was placed on top of the spread outvinegared rice to prepare a round bar-shaped thick-rolled sushi productof 5 cm diameter and 14.5 cm in length (S size) using a sushi rollingmachine for thick-rolled sushi or the like. In this Example, thefollowing ingredients were used as the core as was seen in Example 5; 1)finely chopped shiitake mushrooms which had been reconstituted withwater and then cooked in a broth seasoned with with sugar, soy sauce andsweet cooking sake' (mirin); 2) gourd shavings (kampyo) which had beenreconstituted with water and then cooked in a broth seasoned with sugar,soy sauce and sweet cooking sake' (mirin) and folded in half; 3) Eggseasoned with stock and seasonings such as sugar and soy sauce andcooked into box-shaped Japanese thick omelet, which was then cut into astick-shape with 1 cm width, 1 cm height and 14.5 cm length; and 4)Boiled Japanese mitsuba herb cut in appropriate length.

The thick-rolled sushi product (S size) thus prepared was cut into 8equal pieces and placed in a plastic box having a rectangularparallelepipedonal shape. The box had inner dimensions of 5.5 cm inwidth, 15.0 cm in length and 5.0 cm in depth in its bottom. The boxwhich contained the sushi product was placed in a flexible packing bagmade of a laminated film composed of a polyethylene film as an innerlayer and a nylon film as an outer layer, and the packing bag wasvacuumed by a vacuum pump under a pressure of 40 cmHg and had its mouthheat-sealed. In the vacuumed packing bag, its top portion was depressedand in tight contact with the top surface of the thick-rolled sushiproduct. The volume around the sushi product inside the packing bag (thevolume of the space) was 76 cubic centimeter and the volume ratio of thespace to the sushi product was 0.241. The packing bag, in which thethick-rolled sushi product had been placed together with the plastic boxcontaining the same and which had been vacuumed and heat-sealed, waskept in ethyl alcohol having a temperature of −35° C. as a brine for 45minutes and thereby brine-frozen to obtain a packed frozen thick-rolledsushi product (S size). In the packed frozen thick-rolled sushi product(S size), the plastic packing bag as an outer covering, the plastic boxand the thick-rolled sushi product as contents were frozen together in aunified form. The packed frozen thick-rolled sushi products (S size)were stored at −21.5° C. on average.

In cooking the packed frozen thick-rolled sushi product (S size), a 500W microwave oven was used.

The packed frozen thick-rolled sushi product (S size) in this Examplewas placed in the 500 W microwave oven and thawed by applying heat for 3minutes. 3 minutes, which was the same as the time of thawing by themicrowave oven, after completion of the thawing, the thawed packed sushiproduct was removed from the microwave oven and allowed to stand at roomtemperature for 30 minutes to effect steaming. In this process ofsteaming, temperatures of given portions were measured immediately afterthe thawing, after a lapse of 15 minutes in the steaming (in the Table,referred to simply as “15 minutes later”), and after a lapse of 30minutes for the steaming (in the Table, referred to simply as “30minutes later”). The results are shown in Table 9.

In Table 9, “Portion of the upper surface the end of the rice section”represents temperatures of a portion in the rice section in contact withthe seaweed at the exposed end of the thawed sushi product. Also,“Center portion of the upper surface of the rice section” representstemperatures of a portion on the surface of the seaweed at the center inthe longitudinal direction and in the width direction. Furthermore,“Center portion inside the rice section” represents temperatures of aportion at 2 mm depth from the surface of the seaweed at the center inthe longitudinal direction and in the width direction. Also, “Sushimaterial section” represents temperatures of the sushi material at 2.5cm depth from the surface of the seaweed at the center in thelongitudinal direction and in the width direction.

TABLE 9 Temp. ° C. post-15 post-30 immediately minutes- minutes- afterthawing steaming steaming Portion of the upper surface of the 71.4 29.327.2 end of the rice section Center portion of the upper surface 3.112.9 15.7 of the rice section Center portion inside the rice −1.7 12.115.4 section The sushi materials section −4.6 7.4 12.2

In the thick-rolled sushi product (S size), immediately after thawing,there were differences of 68 to 76° C. between the center portions inthe longitudinal direction and the longitudinal end portion of theshaped rice portion. However, 30 minutes after leaving the sushi productto steam, the temperatures of the shaped rice section and the sushimaterial were about 10° C. or higher, and the overall temperaturedifferences were within a range of 15° C. or less, enabling the thawedthick-roll sushi product to be cut easily with a kitchen knife.

INDUSTRIAL APPLICABILITY

The packed frozen sushi product of the present invention comprises avacuumed and frozen plastic packing bag, an open-topped plastic boxplaced in the packing bag, a laminated metal foil placed on the bottomof the box, and a frozen sushi product having a shaped rice sectiontopped with a sushi material or materials, and these are frozen togetherin a unified form. Accordingly, a space having a volume of one-tenth ormore of that of the sushi product is formed around the sushi product,and when the sushi product is thawed using a microwave oven, flow ofgenerated water vapor is promoted through the space to steam the thawedsushi product. This enables the sushi product to have a roughly uniformtemperature distribution in a relatively short period of time.Accordingly, in the present invention, the packed frozen sushi productcan be thawed and steamed with the water vapor generated in the thawingto enable the thawed sushi product to have desired approximatedtemperatures, and thus the resulting sushi product can be eatendelectably. As described above, the packed frozen sushi product of thepresent invention can be cooked simply and easily, and thusnon-uniformity of temperatures inherent in thawing is solved. By virtueof this, boxed sushi products using a seasonal sushi material ormaterials which have been frozen and stored can delectably be servedirrespective of season.

Further, in the present invention, the packed frozen sushi product whichis hygienically excellent can simply be prepared by placing a sushiproduct in a plastic box, and vacuum-packing and freezing the resultant,and this procedure can be relatively easily performed. Accordingly, thepacked frozen sushi product can be prepared mechanically and thusefficiently on a large scale. Further, it has been considered difficultbut is now realized that the procedure to prepare the frozen packedsushi product may be performed almost mechanically with no substantialmanual intervention and thus easily automatized to enable frozen packedsushi products of high quality to regularly be produced. Therefore, thepresent invention has a high industrial applicability.

1. A microwavable vacuum-packed frozen sushi product comprising: avacuumed and hermetically sealed flexible microwave-safe plastic packingbag which is vacuumed to pressure of 600 mm Hg or lower; an open-toppedplastic box having a top opening and a bottom, the top opening of whichis formed as to be broader than the bottom, placed in said packing bag;in the packing bag, a space being formed around the sushi product in thevacuumed condition, which provides interspaces between the sushi productand the plastic box and between the plastic box and the packing bag; anda frozen sushi product which is formed by at least one shaped form offrozen boiled rice and a sushi material placed on a surface of each formof said at least one shaped form of frozen boiled rice, placed in saidbox; an inner surface of plastic packing bag being in contact withsurface of the sushi material or the boiled rice of said sushi product,and an inner surface of plastic box being in contact with the surface ofthe sushi material and the boiled rice which is different from saidcontacting surface, a space being formed in said plastic packing bagaround said sushi product, said packing bag, said box and said sushiproduct being frozen together in a unified form, the space formed aroundthe sushi product in said packing bag having a volume 0.2 to 0.6 timesthat of sushi product at the time of thawing by means of microwave, andafter thawing, a resulting packed sushi product being able to stand fora period of 15 to 30 minutes without unpacking, allowing water vaporemanating at least from the shaped rice section to flow in a spaceformed between the sushi product and the box, thereby steaming theshaped rice section and the sushi material or materials between 15° C.and 30° C.
 2. The microwavable vacuum-packed frozen sushi productaccording to claim 1, wherein the box is a box or a shallowcylinder-container.
 3. The microwavable vacuum-packed frozen sushiproduct according to claim 1, wherein the microwave-safe plastic packingbag is made of a laminated film composed of a nylon film and apolypropylene film having a thickness larger than that of the nylonfilm.